Just break the cycle

No, he’s not had one-too-many run-ins with new-to-the-road cyclists – John Batten is starting a new series on how process can really help you

By John Batten |

Published:  20 August, 2020

Do you ever get that feeling? You know the one. You turn the key in the ignition, the car cranks, cranks some more, and then some more. You’re willing it to start, but all you're met with is an ever-decreasing RPM as the battery dies along with your hopes for what was going to be a pleasant day.    

Groundhog Day
The sense of doom can often be exacerbated by the fact that a bunch of new bits have been bolted on and the car has been with you all week. I think we have all been there at some point. My key message here though is that it need not feel like this, there is a way to avoid Groundhog Day.
My formative years in this trade were spent working in the family business and it’s the banter between my younger self and my father that reminds me of the path away from Groundhog Day. It went a little like this: “You’ve got a lot to learn son, this game is all about experience” was a familiar message. My dad was right, I did indeed have a lot to learn and you really can’t beat experience. But regardless of how true the message, the regularity at which it and the humorous variants we’re delivered began to grate a little. I needed a witty retort, and then I found one: “You don’t have 30 years of experience dad. You have one year of experience that you’ve repeated 30 times.”
It cut like a knife and although it wasn’t true about him, I can’t help but feel that it’s just so easy to get stuck in a rut and not look for alternative ways to expand our knowledge and make diagnosis just that little bit more enjoyable.
Is that a glimmer of hope I see? Of course it is and all it takes is your will to change and break the cycle. How can the feeling of doom be reduced? Quite simply by improving your process, using the right information and carrying out more tests than are needed on your path to diagnostic stardom.
And the good news is that this is the first article in a series of technical hints, tips and tricks all designed to help you break the cycle. So where shall we start.
I’d normally kick off by looking at your diagnostic process. That being said I’ve covered it in detail previously so I’ll skip it for this article and jump straight into something technical. But just before I do here’s something to remember.
It’s very easy to become consumed by shiny diagnostics. I love cool diag as much as the next geek, but I’ve noticed Pareto’s Law (the 80/20 rule) at work all too often to be tricked into going down that road. Which is why this series will focus on the 20% of the diag that fixes 80% of your problems. let’s get started.
There are many routes you could go down and for this vehicle we’ll assume you’ve no fault codes, your serial data looks good and cranking speed for this petrol car at 250 rpm is on the money. Ultimately you don’t have a lot to go on, so what next?
At this point it’s all about finding diagnostic direction as quickly as possible, you need to find a clue, something that’s out of kilter. And that starts with a 3-step routine that should be second nature for non-start diagnosis and all being well will give you direction.
The question is, do you have a mechanical issue, a fueling problem or an ignition fault?
There’s only one way to tell and that’s to start testing. We could discuss the order we attack this in at length but I’ll normally start with mechanical, then ignition and lastly, fueling. My reasoning being that fueling issues can take a few minutes longer than the other two to test. If I find an issue on my first two tests then I have the initial direction I’m looking for and I’ve shaved a few minutes as well. What fundamental tests should you carry out as part of your non-start routine? That’s straightforward, just grab your scope.
My favourite test at this point is a relative compression test. It’s quick to complete and reveals so much information in such a short amount of time. To set the test up, simply attach your high current clamp around your battery negative cable/s, select a suitable current and timescale, crank the vehicle and you’re off to the races.
Fig.1 shows a good example. Point A being the current to commence rotation and the peak on the subsequent humps is the amount of current to drive a cylinder through its compression stroke. You’ll no doubt have concluded that if you have a single low peak then compression is low on a cylinder. Should your scope support the function, it may be possible to display this test as a bar chart. It can be easier to identify an issue here rather than analysing the current waveform itself.
There’s one key point to remember though. This is a relative test and it may be possible to have more than one cylinder that’s defective and pass the test. I’ll cover this in an article of its own in this series though.
If the relative test shows an issue then you’ll need to carry out a physical compression test for conformation, followed by a cylinder leakage test to discover why. Haven’t found an issue? Then it’s on to your ignition system next.

Ignition Testing
The name of the game is a quick test rather than in depth analysis at this point. The question being: Do you have enough energy to produce a spark for good ignition?
In Fig.2 You’ll see a secondary ignition waveform. Looking at the firing KV at Point B, for sufficient energy to initiate a spark, comparing this on all cylinders and looking for anomalies is a great place to start. Should I have one that’s too low or non-existent, then I’ll be checking powers, grounds, and primary switching at the coil, before considering the possibility of a defective coil. All good? If that’s a resounding yes then let’s take a look at fueling.
‘Those in the know measure flow’ are wise words, and I’ve used flow testing to find many fueling faults. In this instance though, we’re looking for a test that gets us in the ballpark to assess if fuel is being injected. You have a few options. You could:
Opt for a visual inspection of the plugs. Are they wet?
Use a gas analyser pre/post cat.

Do you have HC?
Scope injectors and fuel rail pressure – Does the rail pressure drop while injectors actuated?
No HC, dry plugs, lack of injector actuation and questionable fuel pressure, all give you diagnostic direction and highlight additional testing is required on your fuel sub system.

Found your way?
All being well, your diagnosis now has direction and a path to more specific faults in a given sub-system. Assuming reasonable accessibility and a little practice you’ll often be able to complete those tests in around 30 minutes(ish), which will leave you with at least another 30 minutes to further explore any issues before presenting your findings to your workshop manager.
One thing’s for sure; Having a structured approach to your fault finding, looking for diagnostic direction, with a few familiar tests up your sleeve will reduce your diagnostic time and increase your confidence exponentially. Want to know more? If so then take a look at next issue's article where I’ll be taking a look at some actual non-start issues with detailed test results.

If you’d like to learn how to improve your diagnosis skills then call John on 01604 328500. Auto iQ have a complete technician development programme designed to help your technicians be the best they can be. To join AutoiQ’s online forum go to: autoiq.co.uk/garageowners

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